The transmission of radio signals carrying data in modern wireless communications can be realized based on a number of different communications systems, often specified by a standard. There are increasing requirements for devices which are able to operate to support more than one of these wireless communications systems. Mobile radio receiver devices include analog radio frequency (RF)/intermediate frequency (IF) stages, which are arranged to receive and transmit wireless signals via one or more antennas. The output of the RF/IF stages is typically converted to baseband, where an Analog-to-Digital Converter (ADC) converts incoming analog signals to digital samples, which are then processed for signal detection and decoding of the data in the form of logical values. The ADC may alternatively operate directly at IF, in which case the conversion to baseband is performed in the digital domain. A number of different types of front end processing of the digital samples are known to implement signal detection, including rake receiver processing and channel equalization processing.
In Code Division Multiple Access (CDMA) wireless systems, different physical channels are multiplexed in the code domain using separate spreading sequences.
In the case of orthogonal spreading codewords, the original data symbols can then be effectively separated at the receiver by despreading.
In a Wideband CDMA (WCDMA) cellular system, downlink code multiplexing is performed using Orthogonal Variable Spreading Factor (OVSF) codes. However, the OVSF codewords are orthogonal to each other only under the condition of perfect time alignment. In the presence of multipath propagation, the code orthogonality is lost, and the operation of despreading is affected by Multiple Access Interference (MAI).
CDMA mobile radio receivers conventionally employ a rake processor which relies on the correlation properties of the spreading sequences. A rake processor is described for example in J. G. Proakis, “Digital Communication”, New York, McGraw-Hill, 1995. This type of receiver is subject to performance degradation in the presence of code correlation, if the MAI between code-multiplexed transmission is comparable to the other sources of noise and interference. Under these conditions, a performance advantage may be achieved by attempting to restore the orthogonality between the codes before despreading. The sub-optimality of conventional 3GPP receivers based on rake processing causes a significant performance penalty, especially for downlink data rates increasing from the 384 kbps for WCDMA Release 99 to High Speed Downlink Packet Access (HDSPA) rates of several Mbps. When the code orthogonality is destroyed by multipath, an effective approach is to use channel equalization instead of rake processing.
For example, a Minimum Mean-Square Error (MMSE) chip-level linear equalizer has been shown to provide a significant performance advantage over conventional rake reception, at the cost of an increased implementation complexity. This advantage is especially important for high rate data transmission, as in 3GPP high speed downlink packet access HSDPA. However, channel equalization may not be able to provide superior performance in all possible scenarios. In particular, the use of a channel equalizer does not provide an advantage under single-ray propagation conditions, i.e., in the absence of multipath propagation.
The above limitations generally depend on the particular equalization algorithm under consideration. In the case of a linear MMSE equalizer, in the presence of a non-frequency selective or flat channel response, the equalizer processing still relies on the estimation of the channel impulse response, with a channel estimation error proportional to the number of the channel impulse response samples. In this situation, the use of a rake receiver not only does not correspond to a performance loss caused by MAI, but in fact reduces to a minimum the channel estimation error, relying on the estimate of a single channel tap.
Similarly, in the case of a Least-Squares (LS) equalizer, the receiver performance may be penalized by using the estimation of the channel statistics performed with a dimensionality higher than required in the specific conditions of non dispersive channel, i.e., of channel propagation profile with a single tap.
International Application Publication No. WO 2009/056500 describes a receiver structure capable of selecting the use of rake receiver or equalizer. A number of bases of selection are discussed including identification of the receiver operation under low delay spread channel conditions. In particular, the receiver adaptation can rely on the estimation of the channel root-mean square (rms) delay spread, or on a measure of the channel energy outside a predefined time window. The inventors have determined that a main limitation of these schemes is the difficulty of distinguishing between a single-ray channel profile and a channel profile with low delay spread that may however still benefit from the use of channel equalization. For instance, in the case of a HSDPA receiver, the inventors have demonstrated by internal simulation results and performance tests that the use of a chip level equalizer can still provide a significant performance advantage with respect to the rake receiver for channel profiles with very low delay spread, like, e.g., the Pedestrian A (PA) channel profile of the 3GPP standard.
It is an object of this invention to identify n-ray propagation conditions, which is capable of resolving the above issue.